This invention refers generally to tank level or ullage measurement by reflection of RF or radar signals directed at the surface of the tank contents, and more particularly concerns time/frequency domain processing of calibrate and return signals utilizing digital techniques.
Radar reflective techniques are well known to those skilled in tank level measurement arts. Exemplary of systems of this type is disclosed and claimed in U.S. Pat. No. 4,044,355, hereby incorporated by reference. This patent discloses and claims a sweep frequency, continuous wave radar gauge employing complex analog filtering and processing techniques to improve accuracy of level measurements.
Prior art systems of this type are in fairly common use and have provided reasonably satisfactory operation however, systems of this type suffer from substantial difficulties due to highly complicated systems of removing spurious reflected signals encountered when received signals reflected from the tank content surface are processed. Known gauges of this type suffer from highly complex analog signal processing equipment and resultant degradation of accuracy over periods of time and use.
The system disclosed in this application overcomes many of the shortcomings inherent in presently used systems through the use of time/frequency domain transformations and a novel sweep synchronous measurement and storage of a generated transmit/receive difference signal. The inventions includes determination of a novel "virtual carrier" representation of the differences between transmitted and received tank signals.
It is therefore an object of this invention to provide a radar tank level gauge having improved accuracy through the use of synchronized sweep frequency, and measurement of a generated difference signal having a frequency proportional to tank level.
It is an additional object of this invention to provide a radar tank gauge utilizing time/frequency domain processing of a transmit/return difference signal over a predetermined swept frequency range.
It is a further object of this invention to provide a radar tank gauge wherein a virtual modulated carrier representation of the true transmit/return tank signal and calibrate signal difference is generated.
It is yet an additional object of this invention to provide a radar tank gauge wherein the sweep frequency signal includes gain compensation.
It is a further object of this invention to provide a radar tank gauge wherein returned difference signals include the full range of reflected information, including information sufficient to distinguish the tank bottom or distance limit of the predetermined measurement range.